The proposed study will test and extend the findings from a cross-sectional study that found men who have sex with men (MSM) can be reliably classified into 4 groups based on their pattern of substance use and that the patterns of substance use were associated with both HIV status and with the level of behavioral risk for HIV infection. Men evidencing heavier drug use either through the combined and regular (i.e., weekly or greater) use of club-drugs, poppers, and methamphetamine or the regular use of harder drugs such as heroin and cocaine were more likely to be HIV positive and to evidence higher levels of HIV risk behaviors, such as unprotected receptive anal intercourse, than men who did not use these drugs or used them less frequently. These findings represent one of the first attempts to classify MSM according to their overall pattern of substance use and to test the classification associations with HIV risk behaviors and status through use of latent class analysis. The proposed study builds on this work and the work of other researchers that has found relationships between substance use and risky sexual behaviors for MSM, but using more limited longitudinal or cross-sectional data. Through the application of concepts derived from the life-course perspective (heterotypic continuity, human agency, and social capital), which attempts to explain consistency and changes in health-related behaviors over the lifetime, the proposed study will use the MACS dataset to extend the 4-class model of substance to determine how the pattern of substance use among MSM has changed over time in relation to the trajectory of HIV risk behaviors. Additionally, the study will seek to statistically model the longitudinal associations of individual level factors such as HIV status, health status, and safe-sex fatigue as well as social-contextual factors such as age cohort and the advent of Highly Active Antiretroviral Therapy (HAART), as they affect the dual trajectories of HIV risk behaviors and substance use. Advanced analytic procedures such as growth mixture modeling with parallel processes will be used to analyze the MACS data longitudinally across all measurement points. It is our hope that the findings will lead to a deeper understanding of the inter-relationships among substance use and HIV risk as they change over time, more so than has been possible with temporally limited or cross- sectional data sets. Ultimately our goal is to inform the development of more effective prevention efforts for decreasing substance use, HIV risk behaviors, and the incidence of HIV infection among MSM. Public Health Relevance Statement despite decades of progress in treatment and prevention efforts, HIV/AIDS infections among MSM remains a significant and vexing public health problem, marked by a recent and surprising increase in HIV infection rates as well as the infection rates for other sexually transmitted infections (STIs). The proposed study seeks to enhance understanding of the individual and social-contextual reasons, including patterns of substance use, that underly the recent increase in HIV risk behavior in the MSM population, thereby leading to the development of more effective, contextually sensitive prevention efforts.